Transistor including reduced channel length

ABSTRACT

A transistor includes a substrate. A first electrically conductive material layer, having a thickness, is positioned on the substrate. A second electrically conductive material layer is in contact with and positioned on the first electrically conductive material layer. The second electrically conductive material layer overhangs the first electrically conductive material layer. An electrically insulating material layer, having a thickness, is conformally positioned over the second electrically conductive material layer, the first electrically conductive material layer, and at least a portion of the substrate. The thickness of the first electrically conductive material layer is greater than the thickness of the electrically insulating material layer.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned, U.S. patent application Ser. No.12/986,199, entitled “ACTUATING TRANSISTOR INCLUDING REDUCED CHANNELLENGTH”, Ser. No. 12/986,206, entitled “PRODUCING TRANSISTOR INCLUDINGREDUCED CHANNEL LENGTH”, Ser. No. 12/986,210, entitled “TRANSISTORINCLUDING MULTIPLE REENTRANT PROFILES”, Ser. No. 12/986,218, entitled“ACTUATING TRANSISTOR INCLUDING MULTIPLE REENTRANT PROFILES”, and Ser.No. 12/986,236, entitled “PRODUCING TRANSISTOR INCLUDING MULTIPLEREENTRANT PROFILES”, all filed concurrently herewith.

FIELD OF THE INVENTION

This invention relates generally to semiconductor devices, and inparticular to transistor devices.

BACKGROUND OF THE INVENTION

In semiconductor processing technology, planar substrate surfaces whichare horizontal with respect to a wafer surface are patterned byphotolithographic methods in combination with selective etchingprocesses. During the processing of integrated circuits, reliefs with apronounced topography are formed on the wafer or substrate surface.Typically, this type of relief includes surfaces which are inclined orvertical with respect to the substrate surface. As sizes of integratedcircuits continue to shrink, it is becoming more and more necessary topattern vertical or inclined device surfaces so as to functionallydifferentiate these devices over their vertical extent while stillmaintaining pattern alignment. Examples of these types of semiconductordevices include deep trench capacitors, stacked capacitors, and verticaltransistors.

Currently, it is not possible to put patterns directly on walls whichare vertical with respect to the substrate surface using conventionalphotolithographic techniques. Usually, vertical wall patterning of thisnature is accomplished using a suitable filler material which, whenpartially filling in a trench, acts as a mask for the portions of thewall located underneath while allowing for processing of the walls abovethe filler material. For example, when an oxide is to be depositedexclusively on vertical walls below a filler material, the oxide isfirst deposited or produced over the entire surface of the relief. Therelief or trench is initially completely filled with a suitable fillermaterial. Then, the filler material is recessed back to a depth thatjust covers the desired oxide. After uncovered sections of the oxide areremoved, the remaining filler material is removed.

Alternatively, when an oxide is to be deposited or produced only inupper regions of a vertical wall, an etching stop layer, for example, anitride layer is first provided over the entire surface of the entirerelief pattern. A different material, susceptible to directionaletching, for example, polycrystalline silicon, is used to fill therelief, and is etched back as far as the desired coverage depth of thefinal vertical oxide. After the etching stop layer is removed from theunfilled sections of the walls, an oxide is deposited or generated usinga thermal technique in the uncovered regions. Next, the oxide isanisotropically etched which removes the deposited oxide fromhorizontal. This is followed by removal of the filler material and,then, the removal of the etching stop layer.

There are deposition processes which can be used to deposit thin filmson vertical or inclined surfaces of a substrate relief. However, it isdifficult to control the thickness of the layer deposited. Typically,the thickness of the coating decreases as the depth of the reliefincreases, for example, as the length of the vertical or inclined wallincreases. As such, layers deposited using these types of depositionprocesses have considerable differences in thickness over the length ofthe relief. These types of deposition processes include plasma-enhancedchemical vapor deposition (PECVD) and diffusion-limited deposition ofsilicon oxide using tetraethyl orthosilicate (TEOS).

As such, there is an ongoing need to provide semiconductor devicearchitectures that include patterned vertical or inclined devicesurfaces. There is also an ongoing need to provide manufacturingtechniques capable of processing small device features of semiconductordevices without requiring high resolution alignment tolerances. There isalso an ongoing need to provide higher current semiconductor devices byimproving the series resistance of the device.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a transistor includesa substrate. A first electrically conductive material layer, having athickness, is positioned on the substrate. A second electricallyconductive material layer is in contact with and positioned on the firstelectrically conductive material layer. The second electricallyconductive material layer overhangs the first electrically conductivematerial layer. An electrically insulating material layer, having athickness, is conformally positioned over the second electricallyconductive material layer, the first electrically conductive materiallayer, and at least a portion of the substrate. The thickness of thefirst electrically conductive material layer is greater than thethickness of the electrically insulating material layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description of the preferred embodiments of theinvention presented below, reference is made to the accompanyingdrawings, in which:

FIG. 1 is a schematic cross sectional view of an example embodiment of avertical transistor made in accordance with the present invention;

FIGS. 2 through 8B are schematic cross sectional views of process stepsassociated with an example embodiment of a method of producing thevertical transistor shown in FIG. 1;

FIG. 9 is a graph showing performance I_(d)-V_(d) curve characteristicsfor the transistor shown in FIG. 1; and

FIG. 10 is a graph showing performance transfer characteristics for thetransistor shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present description will be directed in particular to elementsforming part of, or cooperating more directly with, apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown or described may take various forms wellknown to those skilled in the art. In the following description anddrawings, identical reference numerals have been used, where possible,to designate identical elements.

The example embodiments of the present invention are illustratedschematically and not to scale for the sake of clarity. One of ordinaryskill in the art will be able to readily determine the specific size andinterconnections of the elements of the example embodiments of thepresent invention.

Referring to FIG. 1, a schematic cross sectional view of a verticaltransistor 100 is shown. Transistor 100 includes a substrate 110, afirst electrically conductive material layer 120, and a secondelectrically conductive material layer 130. Transistor 100 also includesan electrically insulating material layer 150 and a semiconductormaterial layer 160. An electrode or electrodes 710 and an electrode 810are also included in transistor 100.

Conductive layer 120 is positioned between substrate 110 and secondelectrically conductive material layer 130. A first surface ofconductive layer 120 contacts a first surface of substrate 110 while asecond surface of conductive layer 120 contacts a first surface ofsecond electrically conductive layer 130. Substrate 110, often referredto as a support, can be rigid or flexible.

Second electrically conductive material layer 130 is appropriatelydimensioned (or sized), positioned, or dimensioned and positionedrelative to first electrically conductive material layer 120 to create areentrant profile 170 in transistor 100. As such, it can be said that atleast a portion of second conductive layer 130 defines the reentrantprofile 170 of transistor 100. The reentrant profile 170 shields atleast some of second electrically conductive material layer 130 frommaterial deposited (or coated) using a directional (or line of sight)deposition (or coating) process. Alternatively stated, the secondelectrically conductive material layer 130 itself has a reentrantprofile because a first portion of second electrically conductivematerial layer 130 overhangs a second portion of second electricallyconductive material layer 130.

The second electrically conductive layer 130 is also dimensioned (orsized) and positioned to extend beyond (or overhang) conductive layer120. Alternatively stated, first electrically conductive layer 120 isdimensioned (or sized) and positioned to stop at both ends (in both theleft and right directions as shown in FIG. 1) before both ends of secondelectrically conductive layer 130 stop so that second electricallyconductive layer 130 creates an overhang 180 (also shown in FIG. 4).Alternatively stated, the second electrically conductive material layer130 creates a reentrant profile relative to the first electricallyconductive material layer 120.

Electrically insulating material layer 150 conforms to the reentrantprofile 170 and overhang 180 of transistor 100. Electrically insulatingmaterial layer 150 includes first and second surfaces with the firstsurface being in contact with portions of surfaces of secondelectrically conductive material layer 130, first electricallyconductive layer 120, and substrate 110. Semiconductor material layer160 conforms to electrically insulating material layer 150.Semiconductor layer 160 includes first and second surfaces with thefirst surface being in contact with the second surface of electricallyinsulating layer 150. Distinct (or separate, or different) portions ofthe second surface of semiconductor layer 160 are in contact withelectrode(s) 710 and electrode 810.

The thickness of the first electrically conductive material layer 120 ismore than the thickness of the electrically insulating layer 150 andpreferably equal to twice the thickness of the electrically insulatinglayer 150. The thickness of the first electrically conductively layer120 can also be sized so that it is less than twice the sum of thethicknesses of electrically insulating layer 150 and semiconductinglayer 160. Sizing the thickness of the first electrically conductivematerial layer 120 in this manner allows the conformal coating of theelectrically insulating material layer 150 and, if necessary, a portionof the semiconducting material layer 160 to fill in the overhang 180which ultimately shortens the channel path (or length) and reduces oreven prevents ungated regions in transistor 100.

Electrode(s) 710 includes a third electrically conductive material layer700. When there is more than one electrode 710, different discretediscontinuous portions of third electrically conductive material layer700 form electrodes 710. Electrode 810 includes a fourth electricallyconductive material layer 800. Electrode(s) 710 and electrode 810 arepositioned spaced apart from each other at different locations oftransistor 100. Electrode(s) 710 and electrode 810 can be differentportions of the same material layer. When this happens, the third andfourth electrically conductive material layers 700 and 800 are differentdiscrete discontinuous portions of the same material layer, for example,material layer 700. The material layer, for example, layer 700, ispreferably deposited in a single collimated deposition during whichreentrant profile 170 electrically separates each electrode from theother electrodes such that electrode(s) 710 and electrode 810 areincluded on distinct (different) discontinuous portions of the sameelectrically conductive material layer. Alternatively, the third and thefourth electrically conductive material layers 700, 800 can be distinct(different) material layers that are used to form electrode(s) 710 and810.

Electrically conductive material layers 120 and 130 function as the gateof transistor 100. In some example embodiments of transistor 100, one orboth of electrodes 700 function as the drain of transistor 100 whileelectrode 810 functions as the source of transistor 100. In otherexample embodiments of transistor 100, one or both of electrodes 700function as the source while electrode 810 functions as the drain.

The semiconductor device is actuated in the following manner. Aftertransistor 100 is provided, a voltage is applied between the thirdelectrically conductive material layer 700 and the fourth electricallyconductive material layer 800. A voltage is also applied to the firstelectrically conductive material layer 120 to electrically connect thethird electrically conductive material layer 700 and the fourthelectrically conductive material layer 800. Since first electricallyconductive material layer 120 and second electrically conductivematerial layer 130 are both electrically conductive and in contact witheach other, applying a voltage to one layer, for example, layer 120, isconsidered to be equivalent to applying a voltage to both layers, layers120 and 130, or the other layer, for example, layer 130. The thirdelectrically conductive material layer 700 and the fourth electricallyconductive material layer 800 can be the same material layer or can bedifferent material layers.

The reentrant profile 170 of transistor 100 allows a dimension of thesemiconductor material channel of the transistor to be associated withthe thickness of the second electrically conductive material layer 130,which functions as the gate, of transistor 100. Advantageously, thisarchitecture of the present invention reduces reliance on highresolution or very fine alignment features during the manufacture oftransistors that include small channels.

Referring to FIGS. 2 through 8B, schematic cross sectional views ofprocess steps associated with an example embodiment of a method ofmanufacturing transistor 100 are shown.

Generally described, transistor 100 is fabricated in the followingmanner. A substrate 110 is provided including in order a firstelectrically conductive material layer 120 and a second electricallyconductive material layer 130. A resist material layer 140 is depositedover second electrically conductive material layer 130. Resist materiallayer 140 is patterned to expose a portion of second electricallyconductive material layer 130, shown in FIG. 2. The exposed portion ofsecond electrically conductive material layer 130 is removed using aprocess which tends to create a first reentrant profile in the secondelectrically conductive mater layer 130, for example, plasma etching, toexpose a portion of electrically conductive material layer 120. Theexposed portion of electrically conductive material layer 120 isremoved, shown in FIG. 3. Continued removal of conductive material layer120 creates overhang 180 (which can be referred to as a second reentrantprofile) in which second electrically conductive material layer 130extends beyond first electrically conductive material layer 120, shownin FIG. 4. Alternatively stated, first electrically conductive materiallayer 120 underhangs the second electrically conductive material layer130.

After removal of photoresist material layer 140 (shown in FIG. 5), ifsuch is necessary, substrate 110 and the remaining exposed portions ofelectrically conductive material layers 120 and 130 are conformallycoated with an electrically insulating material layer 150, shown in FIG.6. The thickness of the electrically insulating material layer 150 isless than the thickness of first electrically conductive material layer120, and preferably half the thickness of first electrically conductivematerial layer 120 so that the overhang 180 between the substrate 110and the second electrically conductive material layer 130 can besubstantially filled in. Electrically insulating material layer 150 isconformally coated with a semiconductor material layer 160, shown inFIG. 7. An electrically conductive material layer, for example, materiallayer 700 or material layer 700 and material layer 800, is directionally(or nonconformally) deposited (shown using arrows 900) oversemiconductor material layer 160, shown in FIG. 8A.

The resist material layer 140 can be deposited over second electricallyconductive material layer 130 and patterned in the same process step. Aplasma can be used to remove the exposed portion of the secondelectrically conductive material layer 130 to expose a portion of theelectrically conductive material layer 120 and create reentrant profile170. The same plasma that is used to remove the exposed portion of thesecond electrically conductive material layer 130 can be used to removethe exposed portion of the first electrically conductive material layer120 to create the reentrant profile 180 in the electrically conductivematerial layer 120 if the etch rate of first electrically conductivematerial layer 120 is faster than second electrically conductivematerial layer 130. In some embodiments the first electricallyconductive material layer is wet etched with an etchant which does notetch the second electrically conductive material layer 130 to create thereentrant profile 180.

In some example embodiments, substrate 110 can include more than onematerial layer. The additional material layer(s) is included in someinstances to improve or maintain the structural integrity of substrate110 during the manufacturing process. When substrate 110 includes morethan one material layer, for example, a first material layer and asecond material layer, the fabrication method can include removing thesecond material layer of substrate 110.

Referring back to FIG. 2, vertical transistor device 100 begins with asubstrate 110 that is non-conductive, either in whole or in part withrespect to at least the portion of the substrate that is adjacent toconductive material layer 120 (the top of the substrate 110 as shown inFIG. 2), such that electrical shorting of transistor 100 does not occur.Conductive material layer 120 is applied to (for example, deposited orcoated) onto substrate 110. Conductive material layer 120 functions aspart of the gate of transistor 100 and by its thickness (in the verticaldirection as shown in FIG. 2) defines a length approximately twice theinsulator thickness by its thickness. A second electrically conductivematerial layer 130 is applied on conductive material layer 120.Conductive material layer 130 is a uniform material layer with nopattern. A resist material layer 140 is applied to conductive materiallayer 130. Resist 140 is patterned.

Substrate 110 does not interact appreciably with any of the materiallayers or the processing methods. Substrate 110, often referred to as asupport, can be used for supporting the thin film transistor (alsoreferred to as a TFT) during manufacturing, testing, or use. Thoseskilled in the art will appreciate that a support selected forcommercial embodiments can be different from one selected for testing orscreening embodiments. In some embodiments, substrate 110 does notprovide any necessary electrical function for the TFT. This type ofsubstrate 110 is termed a “non-participating support” herein. Usefulsubstrate materials include organic or inorganic materials. For example,substrate 110 can include inorganic glasses, ceramic foils, polymericmaterials, filled polymeric materials, coated metallic foils, acrylics,epoxies, polyamides, polycarbonates, polyimides, polyketones,poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene)(sometimes referred to as poly(ether ether ketone) or PEEK),polynorbornenes, polyphenyleneoxides, poly(ethylenenaphthalenedicarboxylate) (PEN), poly(ethylene terephthalate) (PET),poly(ether sulfone) (PES), poly(phenylene sulfide) (PPS), andfiber-reinforced plastics (FRP). The thickness of substrate 110 canvary, typically from about 100 μm to about 1 cm.

A flexible support or substrate 110 is used in some example embodimentsof the present invention. Using a flexible substrate 110 allows for rollprocessing, which can be continuous, providing economy of scale andeconomy of manufacturing over flat or rigid supports. The flexiblesupport chosen is preferably capable of wrapping around thecircumference of a cylinder of less than about 50 cm in diameter, morepreferably 25 cm in diameter, and most preferably 10 cm in diameter,without distorting or breaking, using low force as by unaided hands. Thepreferred flexible support can be rolled upon itself. Additionalexamples of flexible substrates include thin metal foils such asstainless steel provided the foils are coated with an electricallyinsulating material layer to electrically isolate the thin filmtransistor. If flexibility is not a concern, then the substrate can be awafer or sheet made of materials including glass and silicon.

In some example embodiments, substrate 110 can include a temporarysupport or support material layer, for example, when additionalstructural support is desired for a temporary purpose, e.g.,manufacturing, transport, testing, or storage. In these exampleembodiments, substrate 110 can be detachably adhered or mechanicallyaffixed to the temporary support. For example, a flexible polymericsupport can be temporarily adhered to a rigid glass support to provideadded structural rigidity during the transistor manufacturing process.The glass support can be removed from the flexible polymeric supportafter completion of the manufacturing process.

The electrically conductive material layers 120 and 130, commonlyreferred to as conductors, can be any suitable conductive material thatpermits conductive material layers 120 and 130 to function as a gate. Avariety of gate materials known in the art are also suitable, includingmetals, degenerately doped semiconductors, conductive polymers, andprintable materials such as carbon ink, silver-epoxy, or sinterablemetal nanoparticle suspensions. For example, the gate electrode caninclude doped silicon, or a metal, such as aluminum, chromium, gold,silver, nickel, copper, tungsten, palladium, platinum, tantalum, andtitanium. Gate electrode materials can also include transparentconductors such as indium-tin oxide (ITO), ZnO, SnO2, or In2O3.Conductive polymers also can be used, for example polyaniline,poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS). Inaddition, alloys, combinations, and multilayers of these materials canbe used. The gate electrode (layers 120 and 130) can be deposited onsubstrate 110 using chemical vapor deposition, sputtering, evaporation,doping, or solution processing

The thickness (the vertical direction as shown in FIG. 2) of the gateelectrode can vary, typically from about 100 to about 10000 nm. As thethickness defines the gate length, the thickness is usually thicker thantwice the thickness of the conformally coated materials in order toreduce the likelihood of electrical shorting in subsequent appliedmaterial layers.

Resist 140 can be a conventional photoresist known in the art such as apolymeric positive acting resist or a negative resist. Resist 140 can beexposed through a mask with a low resolution (>0.1 mm) alignment tosubstrate 110 and developed to yield a pattern of resist. In anotherexample embodiment, the pattern of resist 140 is accomplished using aprinting process, for example, flexography or inkjet printing, thatprints the resist directly in a patterned manner without using a mask.

Referring back to FIG. 3, a schematic cross sectional view of transistor100 material layers during and after material processing are shown. InFIG. 3, second electrically conductive material layer 130, commonlyreferred to as a second conductor, is etched through patterned resist140 to create a first reentrant profile 170. The etchant can be anyorganic or inorganic material which, when used in a suitable etchingprocess, removes the conductive material without substantial attackingresist 400 and provides the reentrant profile 170. First electricallyconductive material layer 120, commonly referred to as a firstconductor, is then removed using a suitable etchant which removes thefirst conductor 120, but has little impact on substrate 110 or theoverlying second conductor 130. As such, the selected etchant oftendepends on the substrate 110, the conductor, 120, or the nonconductor130. Etchant interaction with resist 140 and loss of the resist 140 atthis point is usually of little consequence, since the second conductor130 now acts as a mask. As shown in FIG. 3, the etching process orprocesses used may etch away portions of first conductor 120 and secondconductor 130 such that first conductor 120 and nonconductor 130 havethe same pattern except for the reentrant profile 170 in the secondconductor 130.

Referring back to FIG. 4, selective etching of first conductor 120 iscontinued until the overhang 180 (a second reentrant profile 180) isformed. When etching of first conductor 120 is complete, secondconductor 130 overhangs first conductor 120 which creates a reentrantprofile 180 that allows the dielectric nonconductive material 150 tosubstantially fill in when the dielectric nonconductive material 150 isdeposited using a conformally coating process. Alternatively stated,conductor 120 underhangs nonconductor 130. The remaining conductor 120acts as the conductor which is electrically part of the gate when thesemiconductor device is complete.

Referring back to FIG. 5, at this point, if it is necessary, resist 140is removed. Gentle cleaning can be performed on the material layerstack, if desired, provided that the cleaning process does not removethe reentrant profile 170.

Referring back to FIGS. 6 and 7, schematic cross sectional views of thesemi-conductor device after conformal coating of a dielectricnonconductive material, often referred to as an insulator, and asemiconductor material, respectively, are shown. In FIG. 6, a dielectricnonconductive material 150 is then conformally coated using a conformalcoating deposition process over substrate 110 and the topographicfeature formed by conductive material layers 120 and 130. Applying adielectric nonconductive material 150 using a conformal coating processhelps to maintain the reentrant profile 170. Since the firstelectrically conductive layer 120 is about twice the thickness of thedielectric nonconductive material 150 the reentrant profile 180 can befilled-in and helps to maintain a sharp corner. The dielectricnonconductive material 150 is often referred to as the gate dielectric.Suitable nonconductive materials include strontiates, tantalates,titanates, zirconates, aluminum oxides, silicon oxides, tantalum oxides,titanium oxides, silicon nitrides, barium titanate, barium strontiumtitanate, barium zirconate titanate. As the dielectric materialseparates the gate conductor from the semiconductor material that is tobe applied, it is important that the conformally coated material beprovided with a consistent or uniform thickness at least in the regionwhere the reentrant profile 170 and the gate are located.

Preferred processes for accomplishing conformal coating include atomiclayer deposition (ALD) or one of its derivatives such as spatial ALD(S-ALD) or plasma enhanced ALD (PEALD) because these processes yield auniform thickness coating over or on a highly varying topology. ALD andS-ALD are discussed in more detail below.

In FIG. 7, a semiconductor material 160 is then coated using a conformalcoating deposition process which helps to maintain the reentrant profile170. This conformal coating process can be the same process usedpreviously to coat the dielectric material. Alternatively, the conformalcoating process can be different. As the semiconductor material 160 actsas a channel between electrode(s) 710 and electrode 810 when firstconductor 120 is energized, it is important that the conformally coatedmaterial be provided with a consistent or uniform thickness at least inthe region where the reentrant profile 170 and the gate are located andmore preferable in the areas between electrode(s) 710 and electrode 810including the area where the reentrant profile 170 and the gate arelocated. A preferred process for conformally coating includes atomiclayer deposition (ALD) or spatial ALD (S-ALD), a derivative of ALD.Either process yields a uniform thickness on a highly varying topology.

Atomic Layer Deposition (ALD) is a process which is used to producecoatings with thicknesses that can be considered consistent, uniform, oreven exact. ALD produces coatings that can be considered conformal oreven highly conformal material layers. Generally described, an ALDprocess accomplishes substrate coating by alternating between two ormore reactive materials commonly referred to as precursors, in a vacuumchamber. A first precursor is applied to react with the substrate. Theexcess of the first precursor is removed is removed from the vacuumchamber. A second precursor is then applied to react with the substrate.The excess of the second precursor is removed from the vacuum chamberand the process is repeated.

Recently, a new ALD process has been developed which negates the needfor a vacuum chamber. This process, commonly referred to as S-ALD, isdescribed in at least one of U.S. Pat. No. 7,413,982, U.S. Pat. No.7,456,429, US 2008/0166884, and US 2009/0130858, the disclosures ofwhich are incorporated by reference herein. S-ALD produces coatings withthicknesses that can be considered consistent, uniform, or even exact.S-ALD produces coatings that can be considered conformal or even highlyconformal material layers. S-ALD is also compatible with a lowtemperature coating environment. Additionally, S-ALD is compatible withweb coating, making it attractive for large scale production operations.Even though some web coating operations may experience alignment issues,for example, web tracking or stretching issues, the architecture of thepresent invention reduces reliance on high resolution or very finealignment features during the manufacturing process. As such, S-ALD iswell suited for manufacturing the present invention.

The semiconductor material layer 160, often referred to as asemiconductor, can be any type of semiconductor provided thesemiconductor material can be deposited or coated using a conformalcoating process such as ALD or S-ALD. Examples of suitable semiconductormaterials include zinc oxide, zinc chalcogenides, indium tin oxides,gallium indium tin oxides, gallium tin oxides, cadmium chalcogenides,gallium pnictides, aluminum nictides, germanium, and silicon.

The semiconductor can optionally be doped with other materials toincrease or decrease the conductivity. In some example embodiments, adepletion mode device is desirable, and therefore carriers can be addedthrough the use of dopants. When the semiconductor is a zinc oxide, theuse of an aluminum dopant, for example, increases the electron carrierdensity. In this configuration, the gate is typically used to turn offthe device by making it negative relative to the drain and source.

A compensating dopant can also be used to deplete the intrinsic carrierdensity. When the semiconductor is zinc oxide, the use of nitrogen hasbeen found to decrease the electron carrier density making it lessn-type. In this configuration, the semiconductor can be made to operatein an accumulation mode to turn on the transistor when a positive gatevoltage is applied. These dopants are often added as compounds duringthe growth process but can also be added after the semiconductormaterial layer has been applied using a process such as ion implantationand thermal diffusion.

Referring back to FIGS. 8A and 8B, a schematic cross sectional view ofthe semi-conductor device during directional coating of an electricallyconductive material is shown. After semiconductor material layer 160 hasbeen deposited, the source and drain electrode(s) 710 and electrode 810are deposited using a directional (or line-of-sight) deposition processwhich does not deposit or coat material into the reentrant profile 170.This can also be referred to as a nonconformal deposition process.Examples of suitable directional deposition processes include thermalevaporation, electron beam evaporation, sputtering, or laser ablation.The active channel gap between electrode(s) 710 and electrode 810 ismaintained by the shadow casted by the reentrant profile 170 of secondelectrically conductive material layer 130.

The drain and the source of transistor 100 can be selected from eitherof electrode 700 and electrode 810 with the selection typically beingbased on the application and the characteristics of the contemplateddevice. As shown in FIG. 1, electrode 810 is on the top of the mesaformed by conductor 130 and conductor 120 while electrode(s) 710 is not.As such, electrode 700 and electrode 810 are on different planes. Anynecessary interconnects can be accomplished using conventionaltechniques that are well known in the art, for example, material layerleveling and via feed-through.

Substrate 110, first electrically conductive material layer 120, secondelectrically conductive material layer 130, dielectric nonconductivematerial layer 150, semiconductor material layer 160, electrode(s) 710,or combinations thereof can include one or more layers provided thefunctional aspect of the layer remains unchanged. Additional layers, forexample, leveling layers, barrier layers, adhesion layer, can beincluded in the semiconductor device as long as the function of thelayers described above is preserved.

Experimental Results

A 120 nm material layer of aluminum was deposited via sputtering on a62.5 mm square glass substrate. On top of this, a 460 nm molybdenummaterial layer was coated deposited via sputtering.

A patterned material layer of photoresist was formed by spin coating at1000 rpm Microposit S1805 resist (Rohm and Haas Electronic MaterialsLLC, Marlborough, Mass.) placed on a hot plate for 60 sec at 115 degreesCelsius and then exposed through a glass/chromium contact mask includinglines for 75 seconds on a Cobilt mask aligner (Cobilt model CA-419 fromComputervision Corporation, Sunnyvale, Calif.), using only the edges ofthe silicon substrate as a low resolution or crude alignment. The samplewas then developed for 60 seconds in Microposit MF-319 developer (Rohmand Haas Electronic Materials LLC, Marlborough, Mass.) and rinsed for 5minutes in DI water.

The conductive molybdenum was plasma etched with 0.3 torr SF6 at 200 Wfor 8 minutes using a Technics plasma etcher. The aluminum was thenetched at 60 degrees Celsius with concentrated phosphoric acid for 1.25minutes. The substrate was then rinsed in DI water for 5 minutes, rinsedwith acetone to remove the photo resist, then rinsed in HPLC gradeisopropanol, and then allowed to dry.

The substrate was then conformally coated with a material layer 60 nmthick of aluminum oxide at 200 degrees Celsius using the S-ALD processdescribed in U.S. Pat. No. 7,413,982 and the S-ALD apparatus describedin U.S. Pat. No. 7,456,429 with the organo-metallic precursors trimethylaluminum and water with an inert carrier gas of nitrogen.

The substrate was then coated with a 25 nm material layer of zinc oxideat 200 degrees Celsius using the precursors diethyl zinc andconcentrated ammonia solution and nitrogen as the carrier gas.

The electrodes were applied by evaporation. Aluminum was evaporatedthrough a shadow mask including square holes which ran perpendicular andcompletely cross each line on the substrate. The aluminum was 70 nmthick.

Testing of the transistor was accomplished by using a probe station tocontact the aluminum on top of the line, the aluminum on one side of theline and the chromium gate metal which acts as the gate. Referring toFIG. 9, a graph showing performance I_(d)-V_(d) curve characteristicsfor the transistor is shown. As can be seen in FIG. 9, the drain currentversus drain voltage is very responsive to the gate voltage.

Referring to FIG. 10, a graph showing performance transfercharacteristics for the transistor is shown. As can be seen in FIG. 10,the drain current responds well to the gate voltage, ranging from asmall current of about 10⁻¹¹ amps at a gate of −2 volts to almost amilliamp at a gate of 10 volts for a drain voltage of 1.2 volts. Thegate current, which has very little leakage at all gate voltages, isalso shown

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   -   100 transistor    -   110 substrate    -   120 first conductor    -   130 second conductor    -   140 resist    -   150 a dielectric nonconductive material    -   160 semiconductor    -   170 reentrant profile    -   180 overhang or second reentrant profile    -   700 third electrically conductive material layer    -   710 electrode(s)    -   800 fourth electrically conductive material layer    -   810 electrode    -   900 directional (nonconformal) deposition arrow

The invention claimed is:
 1. A transistor comprising: a substrate; afirst electrically conductive material layer positioned on thesubstrate, the first electrically conductive material layer having athickness; a second electrically conductive material layer in contactwith and positioned on the first electrically conductive material layer,the second electrically conductive material layer overhanging andextending beyond the first electrically conductive material layer, thesecond electrically conductive material layer including a reentrantprofile in which a first portion of the second electrically conductivematerial layer overhangs a second portion of the second electricallyconductive material layer; an electrically insulating material layerconformally positioned over the second electrically conductive materiallayer, the first electrically conductive material layer, and at least aportion of the substrate, the electrically insulating material layerhaving a thickness wherein the thickness of the first electricallyconductive layer is twice the thickness of the electrically insulatingmaterial layer; and a semiconductor material layer that conforms to andis in contact with the electrically insulating material layer.
 2. Thetransistor of claim 1, the semiconductor material layer having athickness, wherein the thickness of the first electrically conductivelayer is less than twice the sum of the thickness of the electricallyinsulating material layer and the thickness of the semiconductormaterial layer.
 3. The transistor of claim 1, further comprising: athird electrically conductive material layer nonconformally positionedover and in contact with the semiconductor material layer.
 4. Thetransistor of claim 3, further comprising: a fourth electricallyconductive material layer nonconformally positioned over and in contactwith the semiconductor material layer.
 5. The transistor of claim 4,wherein the third electrically conductive material layer and the fourthelectrically conductive material layer are different discontinuousportions of the same material layer.
 6. The transistor of claim 1,wherein the substrate is flexible.
 7. A transistor comprising: asubstrate; a first electrically conductive material layer positioned onthe substrate, the first electrically conductive material layer having athickness; a second electrically conductive material layer in contactwith and positioned on the first electrically conductive material layer,the second electrically conductive material layer overhanging andextending beyond the first electrically conductive material layer; anelectrically insulating material layer conformally positioned over thesecond electrically conductive material layer, the first electricallyconductive material layer, and at least a portion of the substrate, theelectrically insulating material layer having a thickness, the thicknessof the first electrically conductive material layer being twice thethickness of the electrically insulating material layer; and asemiconductor material layer that conforms to and is in contact with theelectrically insulating material layer.
 8. The transistor of claim 7,the semiconductor material layer having a thickness, wherein thethickness of the first electrically conductive layer is less than twicethe sum of the thickness of the electrically insulating material layerand the thickness of the semiconductor material layer.
 9. The transistorof claim 7, further comprising: a third electrically conductive materiallayer nonconformally positioned over and in contact with thesemiconductor material layer.
 10. The transistor of claim 9, furthercomprising: a fourth electrically conductive material layernonconformally positioned over and in contact with the semiconductormaterial layer.
 11. The transistor of claim 10, wherein the thirdelectrically conductive material layer and the fourth electricallyconductive material layer are different discontinuous portions of thesame material layer.
 12. The transistor of claim 7, wherein thesubstrate is flexible.